(1) Field of the Invention
The present invention relates to a hybrid relay circuit for switching an AC power supply applied to a load.
(2) Description of the Related Art
Generally, solid state relay circuits have been used for switching an AC power supply applied to a load such as a motor, a signal lamp, an electromagnetic valve (solenoid valve), and the like, which requires a high frequency operation. However, since the above-mentioned solid state relay circuit includes a bidirectional thyristor, it has the following disadvantages:
(1) Due to the peculiar characteristics of the bidirectional thyristor, a reduction of potential of about 1 to 2 Vrms is generated therein, and as a result, when a load current flows through the bidirectional thyristor, a large amount of heat is generated therefrom. Thus, it is difficult to reduce the size of the solid state relay circuit, and as occasion demands, a heat dissipation plate or the like is required.
(2) The bidirectional thyristor has a high cost, thereby increasing the manufacturing cost of the solid state relay circuit.
(3) Since the bidirectional thyristor is weak against surge voltage, it may be erroneously operated or easily broken due to such surge voltage.
(4) When the bidirectional thyristor is turned on, noise is always generated, which may affect the operation of other circuits.